Method of producing toric lens elements with aspheric surfaces

ABSTRACT

A toric lens element is provided with an aspheric surface by first generating a cylindrical surface of revolution on a body of optical material and then honing this surface to aspherize it. While the work is being rotated about the axis of the cylinder, a honing member having a cutting surface of fixed radius is reciprocated back and forth along a path contained within a plane passing symmetrically through the work at an angle to the cylindrical axis. By controlling the curvature of the honing surface and the angle of honing, an aspheric surface is generated which approximates a fourth order sag plot effect. The particular relation is one in which the honing surface is formed with a radius of curvature which is equal to 1/RI cos2 phi +1/RTsin2 phi ; where RI is the instantaneous radius of curvature of the ellipse defined by the intersection of the cylindrical work surface and the plane in which the hone is reciprocated at that point of the ellipse at which the ellipse is intersected by its minor axis, RT is the radius of the cylindrical surface, phi is equal to 90*minus the angle of the plane, RL RIcos2 phi +RTsin2 phi with respect to the axis of rotation, and RL is the radius of the honing surface.

United State Fuller METHOD OF PRODUCING TORIC LENS ELEMENTS WITHASPHERIC SURFACES [75] inventor: David L. Fuller, Atlanta, Ga.

[73] Assignee: Scripto, Inc., Atlanta, Ga.

[22] Filed: Oct. 8, 1971 [21] Appl. No.: 187,605

[52] US. Cl. 51/284, 51/67 Primary Examiner-Donald G. KellyAtt0rneyEdward Taylor Newton, George M. Hopkins and William J Ormsby[57] ABSTRACT A toric lens element is provided with an aspheric sur-SUBSTlTUTE FOR MISSING XR June 19, 1973 face by first generating acylindrical surface of revolution on a body of optical material and thenhoning this surface to aspherize it. While the work is being rotatedabout the axis of the cylinder, a honing member having a cutting surfaceof fixed radius is reciprocated back and forth along a path containedwithin a plane passing symmetrically through the work at an angle to thecylindrical axis. By controlling the curvature of the honing surface andthe angle of honing, an aspheric surface is generated which approximatesa fourth order sag plot effect. The particular relation is one in whichthe honing surface is formed with a radius of curvature which is equalto l/R, cos +l/R,sin where R, is the instantaneous radius of curvatureof the ellipse defined by the intersection of the cylindrical worksurface and the plane in which the hone is reciprocated at that point ofthe ellipse at which the ellipse is intersected by its minor axis, R isthe radius of the cylindrical surface, 4: is equal to 90minus the angleof the plane, R =R,cos +R,sin with respect to the axis of rotation, andR is the radius of the honing surface.

5 Claims, 2 Drawing Figures l M llllllllllllllllllllll:

SEARCH ROOM mam-m 3.739.534

INVENTOR.

[2M0 Z. [011m WM @MM% ATTORNEYS METHOD OF PRODUCING TORIC LENS ELEMENTSWITH ASFI-IERIC SURFACES BACKGROUND OF THE INVENTION In my prior US.Pat. Nos. 3,251,266 and 3,361,512, wide angle photographic objectivesare disclosed which employ toric lens elements and in my copendingapplication Ser. No. 94,830, filed Dec. 3, 1970 a wide anglephotographic lens system is disclosed in which a toric lens elementhaving an aspheric surface in one plane is employed materially toincrease the performance of the system.

The performance increase obtained by the use of aspheric surfaces ontoric lens elements would ordinarily be justified only in unusualcircumstances since the cost involved in forming or generating theaspheric surface by known methods is quite high. If, however, a low costmethod of generating such aspheric surfaces were available, opticalperformance of systems such as are described above could be enhancedmaterially at little or no additional cost.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to themethod of making toric lens elements provided with aspheric surfaces.

Basically, the invention involves the generation of a surface ofrevolution on a body of optical material and then altering this surfacein such fashion as to aspherize it. Specifically, the surface may bealtered according to this invention so as to approximate a fourth ordersag effect very closely. By fourth order sag is meant a surface havingdeformation or sag of the form R+By", where R is a constant, B is aconstant and y is the height or distance from the vertex of the surfacetaken normal to the optical axis.

The alteration of the surface is achieved by reciproeating a honingsurface back and forth along a path contained in a plane whichintersects the axis of the surface, the blank being rotated about thisaxis and the honing surface having a constant or fixed radius ofcurvature. A radial line from the point of intersection between theplane and the axis passes through the vertex of the aspheric surface andthe stroke of the honing surface is adjusted such that its mid point isat such vertex.

BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is an elevational viewillustrating apparatus for performing an aspherizing operation; and

FIG. 2 is a top plan view of the apparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION In the arrangement shown in FIG.I, the base or frame of the machine is indicated by the referencecharacter and mounted on this frame is a suitable bearing support and aspindle supported thereby for rotary motion about the axis 12, thespindle and its associated bearing support not being illustrated inFIG. 1. It is to be understood that a conventional drive for impartingthe rotary motion to the spindle is also provided. Mounted on thespindle is a suitable face plate 14 whose hub 16 is rigidly affixed tothe end of the spindle so that the entire face plate rotates therewith,as is conventional in many machines such as lathes and the like. A bodyof optical material indicated generally by the refernce character 18 isaffixed by adhesive material to a mandrel member 20 which is, in turn,affixed to the face plate 14 in the manner shown.

The bed or frame 10 is provided with a pivot post 22 which pivotallymounts the carrier plate 24 upon which is mounted a motor 26 having ashaft 28 provided with an eccentric or cam 30 thereon. The plate 24carries the upright 32 which is pivotally secured by means of a suitablepivot pin 34 to the lower end of the yoke assembly 36 and a spring 38connected between the yoke 36 and an upright 40 on the plate 24constantly urges the yoke into engagement with the eccentric or cam 30whereby the yoke oscillates as the eccentric 30 is turned by the motor26.

The upper end of the yoke 36 is bifurcated at 40 and carries a pivot pin42 by means of which the transfer arm 44 is pivotally connected to theupper end of the yoke. The transfer arm projects beyond the yoke and isbifurcated at its end 46 and carries a pivot pin 48 for carrying thehoning stone 50. The plate 24 is provided with any suitable hold downbolts or the like by means of which the plate is pivotally adjustedabout the post 22 so as to assume the desired angle a with respect tothe spindle center line 12 as is indicated in FIG. 2. As hereinaftermore particularly described, the honing surface 52 of the stone 50 isformed of a specific radius of curvature and, as will be manifest fromFIG. 2, the honing stone is reciprocated back and forth along a pathcontained within that vertical plane which intersects the vertical planecontaining the axis 12 of the spindle and the axis of the post 22 andwhich plane makes the aforesaid andle a therewith.

It will be appreciated that the honing assembly could be mounted on thecarriage of a lathe but for the purpose of clearly illustrating therelative positioning of the components involved, the plate 24 andrelated pivot post 22 have been shown in the drawing.

In the embodiment of the lens blank element shown, the outer surface 54of the optical material 18 is cylindrical surace of revolution whereasthe inner surface indicated by the reference character 56 has a concavecurvature of uniform radius in a plane containing the axis of the blank.These surfaces are preliminarily prepared in a manner hereinafter setforth.

The plate 24 is set or fixed with respect to the base and the surface 52of the lapping stone is accurately formed to a fixed or constant radiusand is reciprocated in a plane which intersects the cylindrical surface54 making an angle a with respect to the spindle center line 12. Thehoning stone has a length slightly greater than the width of the pathacross the blank 18 and the stroke of the transfer arm 44 is adjusted tobe i onefourth the width across the path as described. Moreover, thestroke of the transfer arm 44 is adjusted so that it extendsequidistantly on opposite sides of the axis of the post 22 so as to bealigned properly with respect to the inner surface 56 of the blankwhereby the vertex of the aspherized outer surface is in a plane normalto the axis of rotation and containing the vertex of the inner surface.

With this arrangement prevailing, the honing stone will alter thesurface along the path width to a circular curvature of radius equal tothat of the honing surface. Under proper circumstances, this alterationmay be made very closely to approximate a fourth order effect, theamount of deformation being a function of the angle at which the lap isset, the radius of curvature of the honing surface, and the radius ofthe circle defined by the vertex of the aspherized surface (toricradius). As a specific example, to generate the surface for the FIG. 2radius of the lens 10 in my aforesaid copending application whereR=40l.478 and B=2.592 l', the honing path was set at an angle a of77.725"; the lap radius R, was 79.5607mm; the instantaneous radius ofcurvature R, was +305.934mm; and R was 76.86808mm.

In order to form the blank 18 of FIGS. 11 and 2, a slab of optical glassis affixed to a suitably shaped mandrel 20 preferably by means of anepoxy cement and then generally cylindrical inner and outer surfaces arefirst formed by securely clamping the assembly to the bed of a drillpress and cutting the cylindrical annulus by means of cylindricalcutting tools, one for the inner surface and one for the outer surface.Each tool takes the form simply of a tube provided with a suitablemandrel for engaging within a chuck of a drill press and with a squaredend edge. The end edge of the tool is brought into contact with the flatface of the glass slab and a slurry of 120 silicon carbide and water iscontinuously or intermittently introduced between the cutting tool andthe glass.

When the rough cylindrical annulus has been thus formed, the blank andits mandrel 20 may be cemented to the face plate 14, again by a suitableepoxy cement and the whole assembly mounted on a suitable lathewhereafter the cylindrical surface 54 is trued as by a diamond cuttingtool. Then the rough contour of the inner surface 56 is formed by adiamond wheel operating in conjunction with a tracer attachment mountedto the lathe carriage as solidly as possible. The rough grindingoperations take place in the presence of a continuous flooding withcoolant solution which preferably is a mixture of gallons of water andone cup of sal soda.

After the rough grinding operations of the aforesaid, surface honing iseffected for the outer surface as described hereinabove and then theinner surface is honed whereafter both surfaces are ready for polishing.The honing operations are, in general, performed along the lines whichhave been developed previously in conjunction with precision machiningof bearing raceways, excepting the specific aspherizing honing describedhereinabove which departs therefrom in the manner specified. The honingis carried out using suitable honing sticks and abrasive grit in grades400, 600, 800 and 1,000, sequentially in that order whereafter thesurfaces are ready for polishing.

The polishing operation proceeds along the lines of the honing exceptthat a polishing lap is prepared by first coating the lap support with amedium soft pitch and then coating this pitch with beeswax. Whereafter,the lap is warmed in hot water and while still pliable is pushed intothe surface of the workpiece which has been coated with a mixture ofpolishing compound and water and the operation thereafter is similar tothe honing operation.

It should be obvious that the surface of revoltuion generated on theblank need not be cylindrical, although calculation with respect to acylindrical surface are required in order to obtain the value of R,. For

til

practical reasons, however, the generated surface is best formed as acylinder in order to assure a more practical and efficient honingoperation.

What is claimed is:

l. The method of making a toric lens element having an aspheric surface,which comprises the steps of:

a. shaping a body of optical material into a blank hav ing at least onesurface of revolution;

b. forming a honing member having a honing surface of selected radius ofcurvature;

c. rotating said blank about the axis of revolution;

and, during step (c),

d. aspherizing said one surface by engaging said honing surface againstsaid surface of revolution and effecting relative reciprocationtherebetween back and forth along a path contained in a plane passingthrough said blank and intersecting said axis at a selected angle.

2. The method of making a toric lens element having an aspheric surface,which comprises the steps of:

a. shaping a body of optical material into an annular blank havingconcentric inner and outer surfaces;

b. forming a honing member having a curved honing surface of constantradius;

c. rotating said blank about the common axis of said concentricsurfaces; and, during step (c),

d. engaging said honing surface against one of said surfaces andeffecting relative reciprocation therebetween back and forth along apath contained in a plane passing through said blank and intersecitngsaid common axis at a selected angle.

3. The method of making a toric lens element having an aspheric surface,which comprises the steps of:

a. shaping a body of optical material into an annular blank havingconcentric inner and outer cylindrical surfaces;

b. forming a honing member having a convex honing surface;

c. rotating said blank about the common axis of said cylindricalsurfaces; and, during step (c),

d. engaging said honing surface against said inner cylindrical surfaceand effecting relative reciprocation therebetween back and forth along apath contained in a plane passing through said blank and intersectingsaid common axis at a selected angle.

4. The method according to claim 2 wherein said plane intersects saidblank along a plane of symmetry thereof, said plane and said outersurface defining an elliptical curve having a selected instantaneousradius of curvature at the point of intersection between said curve andits minor axis, and said honing surface being formed to said selectedinstantaneous radius of curvature.

5. The method according to claim 3 wherein said plane intersects saidblank along a plane of symmetry thereof, said plane and said innersurface defining an elliptical curve having a selected instantaneousradius of curvature at the point of intersection between said curve andits minor axis, and said honing surface being formed to said selectedinstantaneous radius of curvature.

1. The method of making a toric lens element having an aspheric surface,wHich comprises the steps of: a. shaping a body of optical material intoa blank having at least one surface of revolution; b. forming a honingmember having a honing surface of selected radius of curvature; c.rotating said blank about the axis of revolution; and, during step (c),d. aspherizing said one surface by engaging said honing surface againstsaid surface of revolution and effecting relative reciprocationtherebetween back and forth along a path contained in a plane passingthrough said blank and intersecting said axis at a selected angle. 2.The method of making a toric lens element having an aspheric surface,which comprises the steps of: a. shaping a body of optical material intoan annular blank having concentric inner and outer surfaces; b. forminga honing member having a curved honing surface of constant radius; c.rotating said blank about the common axis of said concentric surfaces;and, during step (c), d. engaging said honing surface against one ofsaid surfaces and effecting relative reciprocation therebetween back andforth along a path contained in a plane passing through said blank andintersecitng said common axis at a selected angle.
 3. The method ofmaking a toric lens element having an aspheric surface, which comprisesthe steps of: a. shaping a body of optical material into an annularblank having concentric inner and outer cylindrical surfaces; b. forminga honing member having a convex honing surface; c. rotating said blankabout the common axis of said cylindrical surfaces; and, during step(c), d. engaging said honing surface against said inner cylindricalsurface and effecting relative reciprocation therebetween back and forthalong a path contained in a plane passing through said blank andintersecting said common axis at a selected angle.
 4. The methodaccording to claim 2 wherein said plane intersects said blank along aplane of symmetry thereof, said plane and said outer surface defining anelliptical curve having a selected instantaneous radius of curvature atthe point of intersection between said curve and its minor axis, andsaid honing surface being formed to said selected instantaneous radiusof curvature.
 5. The method according to claim 3 wherein said planeintersects said blank along a plane of symmetry thereof, said plane andsaid inner surface defining an elliptical curve having a selectedinstantaneous radius of curvature at the point of intersection betweensaid curve and its minor axis, and said honing surface being formed tosaid selected instantaneous radius of curvature.